1. Field of the Invention
The invention relates to tracking control in magnetic recording media. More particularly, the invention relates to a servo pattern of the intermittent type for improved tracking control.
2. Description Relative to the Prior Art
In the following description of the relevant prior art, reference is made to FIG. 1 of the accompanying drawings, which illustrates the envelopes of prior art servo burst signals of the intermittent type and corresponding playback signals.
With reference to the magnetic recording art, tracking is the process of keeping a magnetic head, either record or playback, on a particular track. The purpose of tracking control is to adjust the position of the magnetic head relative to the track or vice versa. In a recording mode, tracking control serves to maximize useable recording area without overwrite of existing record tracks. In a playback mode, tracking control functions to maintain a playback head aligned with a record track for maximum signal-to-noise output.
Tracking control is particularly advantageous for magnetic recording apparatus in which the density of tracks is very high. When playback occurs on apparatus other than that which was used for recording, tracking control has been found to be even more advantageous because of mechanical tolerance build-up inherent in different apparatus, variations in magnetic medium, either tape or disk, and other secondary tolerances.
In positioning a magnetic head over a selected track, commonly the head is locked into direct coincidence with the track by means of a closed-loop head-positioning servomechanism. The "servo" utilizes information prerecorded on the magnetic medium. It has been an accepted practice, for either disk or tape, to use either one or more tracks exclusively for permanently recorded clock and servo signals or to use data tracks characterized by the presence of so-called "embedded servo" tracking information, preferably in dedicated areas at locations interleaved with or disposed between data-recording regions along a data track. In the former, a completely separate magnetic head is dedicated exclusively to clocking and/or servo activities, whereas in the latter, the head serves both servoing and data playback activities.
Commonly, servo information is recorded on opposite sides of a reference centerline. If centered directly over the reference centerline, a magnetic head reads two signals of equal amplitude; if off-center, the amplitude of one signal played back exceeds that of the other. The closed-loop head-positioning servo serves to drive the magnetic head in a direction so as to equally match the respective amplitudes of the two servo signals played back, thereby centering the head over the reference centerline.
Of particular interest, insofar as the teachings of the present invention are concerned, is servo information of the intermittent type. That is, a servo pattern having gaps between successive time-limited servo signal bursts. U.S. Pat. No. 4,048,660 shows various servo patterns of this type.
We have found that playback signals, corresponding to servo patterns of the intermittent type, include a spurious component at the beginning and the end of each servo signal burst, i.e, in the gaps between adjacent bursts. Timelines (a) and (b) of FIG. 1 show, respectively, a servo signal of the intermittent type and the corresponding playback signal with its spurious components. More specifically, we have found that the servo signal played back has a spurious positive-going component, denoted A in FIG. 1, prior to the leading edge of each servo burst and either a spurious negative-going component, denoted B, or a spurious positive-going component, denoted C, following the trailing edge of each burst. The polarity of the trailing-edge component depends on whether the servo signal burst includes an even or an odd number of cycles.
These spurious components, either positive or negative-going, cause the head-positioning servo to produce a tracking error signal that is functionally related to the magnitude and the direction of the spurious components played back. Accordingly, with a servo pattern of the intermittent type, the head-positioning servo is unable to continuously maintain a track-following magnetic head in precise alignment with the reference centerline.